The present invention relates in general to well drilling equipment and particularly to hydraulic jars for releasing drill strings or tools trapped in well bores.
As is well known in the art, during the drilling of a well, the drill pipe or well tools sometimes become lodged or stuck in the well bore. To assist in the recovery of a stuck tool, a hydraulic jar is commonly included in the drill string. As is well known in the art, hydraulic jars operate on the principle of a hydraulically delayed longitudinally moving hammer operatively coupled to the drill pipe, the hammer being released after a time delay to strike an anvil operatively attached to the parts stuck in the well bore. To provide the longitudinal movement and the resulting impact forces, tension is applied to the drill string via the usual hoisting equipment. The initial relative longitudinal movement in the jar is retarded by a hydraulic mechanism to permit the desired tension to be applied.
A typical hydraulic mechanism for providing the time delay in the jar comprises a piston moving in a bore to compress a hydraulic fluid in a chamber, the delayed rate of movement of the piston being controlled either by a fluid metering orifice bypassing the piston or alternatively by sizing the piston relative to the bore so that the fluid leaks therebetween at a selected rate. The jar is provided with a release bore of larger diameter than the compression bore to provide a relatively large fluid bypass and sudden release of the piston in the chamber. The compression of the hydraulic fluid produces large magnitude forces within the jar. The tension which can be applied to the jar is often limited by the capability of the jar notwithstanding the hydraulic forces produced therein. These forces sometimes develop a pressure of well in excess of 40,000 lbs. per square inch.
While metering orifices bypassing the piston as described above are well known and quite widely accepted, there is the danger of the metering orifice becoming partially or fully blocked by foreign matter during the course of operation thus impairing or disabling the jar. When a well tool is so stuck in the well bore that tension alone will not release it it is vital that a jar, if used, be in perfect working order. A jar that has failed is of utterly no use and it is probable that the stuck tool will be lost if the drilling string breaks under tension.
The present invention is concerned with the second variety of hydraulic jar referred to above, i.e. one wherein the necessary time delay is provided by sizing the piston relative to the compression bore so that the fluid leaks therebetween at a selected rate during the jarring stroke.
In order to provide for uniformity and consistency in the time delay in a jar of this nature it is of course well known that the outside diameter of the piston must be carefully selected in relation to the inside diameter of the compression bore of the jar. The effect on the time delay of wear on these surfaces and of any changes in surface shape which may occur in the piston as a result of the pressures encountered during use are quite well known and have been dealt with elsewhere. However, there is another vital area of concern and this comprises the surfaces of the inner mandrel and the piston which must come into sealing engagement with one another to prevent bypass of hydraulic fluid therebetween during the jarring stroke. It has been found that unless these surfaces are extremely accurately machined that some leakage therebetween will occur thus making it difficult to maintain a uniform release time from one jar to the next. In addition, during normal use, a certain amount of wear of the contacting surfaces occurs which, particularly when coupled with the presence of any foreign materials in the hydraulic fluid, causes wear and degeneration of the mating surfaces such that the time delay period will vary during normal usage.